With the advent of the microprocessor, a tremendous control industry has developed wherein the microprocessor is employed as a central process unit for a digital data processing system. Such digital systems generally employ a frame or rack for supporting a plurality of generally parallel, card like modules, each of which contains its own digital circuits and components. These cards or modules are generally printed circuit boards with a number of attached components, many of which are IC chips. An array of terminals adjacent an edge of the module is used to interconnect the module with conductors in the rack or frame.
The total digital system, including processor, memories, I/O connections and peripheral accessories, is joined together in an architecture generally determined by the supporting frame. To facilitate interchangeability and maintenance, the frame or rack often includes a motherboard arrangement wherein the bus network itself is permanently located in the supporting frame. By inserting the cards into receptacles on the racks, the terminal arrays on the edge of the cards coact with motherboard terminals to connect the individual modules or cards into the control system. Although this assembly concept is simple, complexity is created by the large number of component and system manufacturers. The scheme by which different suppliers use the permanent bus network is often different. There are many ways to interconnect the separate cards or modules and to apply digital data to the individual conductors in the motherboard of the frame to control the processing of digital data. Consequently, interconnection of the various cards with the processor on one of the cards and communication between the system itself and external equipment demand that the architecture be matched. To provide some uniformity, manufacturers of the processors themselves have developed certain bus arrangements wherein the bus networks include a large number of individual conductors, each of which is assigned a particular function generally associated with the scheme selected by the manufacturer. These bus networks provide a certain uniformity from one system to another as long as the scheme determined by the processor manufacturer is followed. Such attempts to provide uniformity in the industry have not been successful. One major difficulty is that the use of preassigned conductors in a network places drastic constraints on the support frame used in supporting the several cards or modules, the number of receptacles within that frame determine the extent to which the system can be expanded. This provides hesitation on the part of the consuming public in that a major investment may be outdated in a short period of time or, as an alternative, the customer can not maintain state of art technology for its particular control system. This difficulty comes even more pronounced when a customer decides to install additional digital processing systems. Each system must employ its own frame even though it is possible that prior systems used at the same site have capacity which is not employed. Thus, the industry is constrained by lack of standardization and, more importantly, by the inability of existing bus networks to allow flexibility in the number of cards and the number of individual systems mountable in a given frame structure.